It was in the last two or three years that he started to lose his pace. He started to dribble less around the time Mourinho took over Madrid, but that had nothing to do with him losing pace. In his first season at Madrid he was great to watch and scored lots of excellent and exciting goals (both from long range and goals that came from dribbles).
This is a video of his first season at Madrid.
Once Mourinho came to the club (the 10/11 season) it was all about efficiency and Ronaldo reduced the amount he took on players, perhaps in a bid to lessen the strain on his body so that he was able to play more matches throughout the season (when Mourinho came to the club the number of matches Madrid played in a season increased greatly due to Madrid going much further in both the CL and the Copa) as well as Madrid playing very fast, counter-attacking football under Mourinho and he probably wanted players to release the ball quickly rather than attempt to take on other players so as to not slow down the counter-attacks. After Mourinho took over, he has still produced lots of moments of individual brilliance, but not as much as he used to due to him changing his game. That said, if he continued to play like he did in 09/10 and did not change his game, I do not think he would still be playing at a very high level like he currently is. I do not think his body would have lasted as long if he kept playing that way.